The investigator proposes to study the phenotype and function of fine T cell subsets in normal adults, neonatal and pediatric samples, BMT/stem cell transplant patients, chemotherapy/irradiation treated patients, and HIV-infected patients before and after effective anti- viral therapy. The flow cytometry technology available in this lab allows the simultaneous 10-color, 1 parameter analysis of cells that can more clearly identify subsets of T cells and allow the unraveling of the considerable confusion and controversy surrounding the AIDS immunophenotyping literature. The aims are to determine the parameter that define functionally important T cell subsets, define the functional capacity of the subsets, and study the dynamic equilibrium of the subsets in clinically relevant populations. The data obtained will potentially be useful for elucidating the functional capacity of rare T cell subsets and for the monitoring of T cell reconstitution during anti-viral therapy.